Mobile communications, such as mobile telephony, is a technology that is continuously gaining an increased user base. Similarly, as new technologies emerge, they will co-exist with older standards. Furthermore, different geographical areas have different communication standards and spectrum planning. As a User Equipment (UE), such as a mobile telephone is switched on it searches for available network carriers according to a stored history list of Radio Access Technologies (RAT) and carriers. This list is matched against any signal that is received at a present location and allows the UE 100 to connect to one of the network carriers that are in the list. To maintain a high quality connection, a user equipment (UE) is adapted to perform Radio Resource Management (RRM) related measurements. More specifically, the UE performs measurements on its serving cell and neighboring cells (potentially on different frequencies and/or Radio Access Technologies) to determine a received signal power (e.g. WCDMA RSCP (Wideband Code Division Multiple Access Received Signal Code Power), LTE RSRP (Long Term Evolution Reference Signal Received Power) both defined in the 3GPP (3Rd Generation Partnership Project) standard) and/or quality (e.g. WCDMA EcN0, LTE RSRQ (Reference Signal Received Quality) also defined in 3GPP) of a signal from a base station. These measurements may, for example, be used to determine whether a handover from a first base station to a second base station should be executed.
FIG. 1 shows a schematic view of a user equipment (UE) 100, such as a mobile communications device, for example a mobile telephone present in a general network environment. The UE 100 is schematically illustrated in FIG. 1 and it should be noted that a UE 100 comprises various functional components, such as a controller, a memory, a user interface and a radio interface to name a few, that are not illustrated in FIG. 1. Such functional components will be described further on with reference to FIGS. 2 and 3. The UE 100 comprises an apparatus 110 comprising a first layer Layer 1 120 and a third layer Layer 3 130. The first layer 120 and the third layer 130 are, in one embodiment, part of a communication model, such as an Open Systems Interconnection (OSI) model. In FIG. 1 a first base station 160 and a second base station 165 are shown. The first base station 160 and the second base station 165 are adapted to each emit a radio frequency signal 150, 155. The first layer 120 of the apparatus 110 is operably connected to a radio antenna 140 for receiving a radio signal 150, 155 from a base station 160, 165 for performing the measurements. In one embodiment the communication model is at least partly comprised in a radio frequency interface.
Depending on the state of the UE 100 (e.g. Idle or Connected), these measurements may be used internally to decide if a cell reselection should be performed and if so, the proper cell to reselect to. Alternatively, the measurements may be reported to the network (NW), through a base station 160, 165, wherein decisions are taken whether the UE 100 shall be handed over to another cell or not and if so, the proper cell to hand over to is selected. In a typical scenario, should a second signal 155 from a second base station 165, that is stronger than the first signal 150 from the first base station 160 currently selected, be detected, a hand over to the second base station 165 having the stronger signal 155 might be performed. However, if the second signal 155 is only strong for a short while, perhaps as when a vehicle carrying a UE 100 passes a short-range base station, the second signal 155 will only be strong for a short time. This will result in that the UE 100 will be handed over to the second base station 165 for only a short time and as the second signal 155 grows weaker the UE 100 will be handed over back to the first base station 150. This causes an unnecessary use of system resources such as bandwidth, computational power and also battery power of the UE 100. In order to avoid such a trigger-happy system, the aforementioned measurements are often smoothed by filter(s) before they are acted upon. As an example, in 3GPP (both for WCMDA and LTE) there are two filters defined; one implementation-dependant Layer 1 filter and one specified Layer 3 filter that filters the output from the Layer 1 filter before any action is taken.
Contemporary filters used for RRM-related measurements may be based on traditional filters, for example Finite Impulse Response (FIR) filters and Infinite Impulse Response (IIR) filters known in the prior art. As an example, one such generic Layer 3 filter is specified in 3GPP (WCDMA 25.331, LTE 36.331) to be applied on cellular measurements before a report is submitted to the NW by the UE 100. The filter is specified as:F(n)=(1−a)F(n−1)+a M(n)where:    F(n) is the updated filtered measurement result    F(n−1) is the old filtered measurement result    M(n) is the latest received measurement result from physical layer measurements    a is a coefficient configured by the NW (typical value range 0 . . . 1)
The measurement M(n) is in turn a Layer 1 filtered version of the received signal strength. The L1 filter is implementation dependent and could either be a FIR filterM(n)=(1/N)sum_{i=0}^{N−1}Y(n−i)Or an IIR filterM(n)=(1−b)M(n−1)+b Y(n)Where Y is the signal strength/quality estimate and N is the number of samples that M(n) is filtered over for a FIR implementation. The filter coefficient b is the corresponding time constant in case of IIR implementation. In a typical embodiment, b or N as applicable may be chosen such that the time constant is around 200 ms.
In FIG. 1 an example relationship between Y(n), M(n) and F(n) is illustrated with the arrows marked Y, M and F respectively.
There is a trade-off for a UE on how much it should filter its RRM related measurements. The more or harder a UE filters its measurements (e.g. the filter coefficients a and/or b (or 1/N) are set to low values), the less responsive it will be to surrounding radio environments with the possibility of experiencing loss of coverage, miss incoming calls etc. On the other hand, the less or softer a UE filters its measurements (e.g. the filter coefficients a and/or b (or 1/N) are set to high values), the more responsive it will be to the surrounding radio environments and the UE will react faster to changes or, in other words, the UE will be more inclined to changing cells (trigger-happy), flooding the network with measurement reports, etc.
As a result, in the 3GPP L3 filter example above, the network configures the filter (i.e. configures the coefficient a) in a generic manner that must serve “okay”, that is provide an acceptable signal quality, in all scenarios. This will typically give rise to problems in specific scenarios, where the “average” or generic choice of the filter coefficient or parameter a is not satisfactory. According to the standards the coefficient a can assume values in the range 0 to 2, commonly in the range 0 to 1, a typical value being a=0.7.
The American patent application US2011103350 discloses a cellular communication system that has an air interface divided into frames, each consisting of sub-frames at least two of which are synchronization sub-frames. For each cell, different cell-related synchronization signals are transmitted to user equipments (UEs) in different synchronization signal sub-frames. The UE detects cell identities of first and second cells. Weights then control generation of weighted handover measurements made from the first cell's synchronization signals received during synchronization sub-frames, wherein each of the weights is a function of the cell identity of the first cell, the cell identity of the second cell, and which ones of the first and second cells' cell-related synchronization signals are transmitted in the respective one of the plurality of synchronization sub-frames during which the weight is applied. A filtered handover measurement, upon which a handover decision can be made, is generated from the weighted handover measurements. This system thus adapts the weights for the synchronization signals depending on the CellID only.
There is thus a need for a method, an apparatus to be implemented in a UE and a computer program product that improves RRM measurement filtering. For example, there is a need for a method, an apparatus to be implemented in a UE and a computer program product for alleviating the above mentioned short comings with the Network decided L3 filter parameter choice.